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than 4 feet below ground surface. The vault shall extend from the top of the annular <br /> seal to at least ground surface. <br /> The use of subsurface vaults to house the top of water wells below ground surface is <br /> rare and is discouraged due to susceptibility to the entrance of surface water, <br /> pollutants,and contaminants. Where appropriate,pitless adapters should be used in <br /> place of vaults. <br /> B. Sealing Conditions. The following requirements are to be observed for sealing the annular ■ <br /> space. <br /> 1. Wells drilled in unconsolidated.caving material. An'oversized'hole,at least 4 inches <br /> greater in diameter than the outside diameter of the well casing, shall be drilled and <br /> a conductor casing temporarily installed to at least the minimum depth of annular seal <br /> specified in Subsection A, page 14, above. Permanent conductor casing may be used <br /> if it is installed in accordance with Item 3, page 16, below, and Item 5 (page 32 of <br /> Bulletin 74-81)and if it extends at least to the depth specified in Subsection A,above. <br /> One purpose of conductor casing is to hold the annular space open during well drilling <br /> and during the placement of the well casing and annular seal. <br /> Temporary conductor casing shall be withdrawn as sealing material is placed between <br /> the well casing and borehole wall,as shown in Figure 4A(page 31 of Bulletin 74-81). <br /> Sealing material shall be placed at least within the interval specified in Subsection A, <br /> above. The sealing material shall be kept at a sufficient height above the bottom of <br /> the temporary conductor casing as it is withdrawn to prevent caving of the borehole <br /> wall. <br /> Temporary conductor casing may be left in place in the borehole after the placement <br /> of the annular seal only if it is impossible to remove because of unforeseen conditions <br /> and not because of inadequate drilling equipment, or if its removal will seriously <br /> jeopardize the integrity of the well and the integrity of subsurface barriers to pollutant <br /> or contaminant movement. Temporary conductor casing may be left in place only at <br /> the approval of the enforcing agency on a case-by-case basis. <br /> Every effort shall be made to place sealing material between the outside of temporary <br /> conductor casing that cannot be removed and the borehole wall to fill any possible <br /> gaps or voids between the conductor casing and the borehole wall. At least two inches <br /> of sealing material shall be maintained between the conductor casing and well casing. <br /> At a minimum,sealing material shall extend through intervals specified in Subsection <br /> A, above. <br /> Sealing material can often be placed between temporary conductor casing that cannot <br /> be removed and the borehole wall by means of pressure grouting techniques, as <br /> described below and in Appendix B (page 67 of Bulletin 74-81). Other means of <br /> placing sealing material between the conductor casing and the borehole wall can be <br /> used, at the approval of the enforcing agency. <br /> Pressure grouting shall be accomplished by perforating temporary conductor casing <br /> that cannot be removed,in place. The perforations are to provide passages for sealing <br /> material to pass through the conductor casing to fill any spaces and voids between the <br /> casing and borehole wall. Casing perforations shall be a suitable size and density to <br /> allow the passage of sealing materials through the casing and the proper distribution <br /> -15- <br />